The present disclosure relates to a laser diode assembly.
Recently, in the advanced scientific region researches using laser light with the pulse time on the attosecond time scale or on the femtosecond time scale, the ultrashort pulse and ultrahigh power laser is actively used. The ultrashort pulse laser commands scientific interest to clarify ultrafast phenomenon on the picosecond time scale or femtosecond time scale. In addition, regarding the ultrashort pulse laser, application researches on practical realization such as microfabrication and two-photon imaging by using high peak power have been actively made. Further, the high power and ultrashort pulse laser diode device that is composed of GaN compound semiconductor and that has light emitting wavelength of 405 nm band has been expected to be a light source for a volumetric optical disc system promising as a next generation optical disc system displacing the Blu-ray optical disc system, has been expected to be a light source needed in the medical field, the bio imaging field, and the like, or has been expected to be a coherent light source that covers a whole visible light region.
As the ultrashort pulse and ultrahigh power laser, for example, titanium/sapphire laser is known. Such titanium/sapphire laser is an expensive and large solid laser light source, which is a main factor to inhibit spread of the technology. If the ultrashort pulse and ultrahigh power laser is realized with the use of a laser diode or a laser diode device, substantial miniaturization, price reduction, low power consumption, and high stability are able to be realized, which is expected to become a breakthrough for promoting its wide usage in these fields.
A laser diode assembly having a whole semiconductor structure as the foregoing high peak power and picosecond pulse light source in 405 nm band generally has a MOPA (Master Oscillator and Power Amplifier) structure. Specifically, the laser diode assembly is composed of a laser diode that generates a picosecond pulse and a semiconductor optical amplifier (SOA, or a semiconductor laser amplifier) that amplifies the generated picosecond pulse (semiconductor optical amplifier (SOA)). Specific examples of pulse light source that generates a picosecond pulse in the MOPA structure include a mode-locked laser diode assembly having an external resonator.
The mode-locked laser diode assembly is realized by, for example, a multielectrode mode-locked laser diode device and an external resonator arranged on its light axis. One end face of the multielectrode mode-locked laser diode device often includes a high reflecting coating layer, and also functions as a mirror opposed to the external resonator. By arranging a wavelength selection device such as a diffraction grating and a bandpass filter composed of a dielectric multilayer film, oscillation wavelength is able to be selected.
In the case where the diffraction grating is used as the wavelength selection device, the external resonator is composed of the diffraction grating, primary diffracted light is returned to the laser diode device, and thereby oscillation wavelength is able to be selected. Such arrangement is known as Littrow configuration or Littman configuration, and is used for a continuous oscillation wavelength-variable laser (for reference, see Japanese Unexamined Patent Application Publication No. 2001-284716; “Electronics letters,” vol. 33, No. 16, p. 1387, 1997, Heim et al; and “Optics letters,” vol. 24, No. 22, p. 1573, 1999, Struckmeier et al). Meanwhile, in the case where the bandpass filter is used as the wavelength selection device, the bandpass filter is arranged between the laser diode device and the external resonator, and thereby wavelength selectivity is obtained (for example, see Japanese Unexamined Patent Application Publication No. 2002-164614).